The present invention relates to a system for transmitting the power of an engine to four wheels of a four-wheel drive vehicle.
In a conventional four-wheel drive vehicle having a central differential, which is called a full time four-wheel drive vehicle, the central differential operates to uniformaly split the torque of the engine for the front wheels and rear wheels of the vehicle.
It is known that the distribution ratio of torque influences driveability, steerability, stability, starting characteristic and other factors. For example, the torque is split in accordance with the distribution of load on the front and rear wheels of the vehicle for good traction. If the torque split is controlled at cornering, a stable cornering characteristic can be obtained. Accordingly, the understeer encountered in a vehicle driven by front wheels and the oversteer encountered in a vehicle driven by rear wheels can be restrained. Further, on slippery roads, the distribution of the torque to the rear wheels can be increased so as to ensure safe driving without locking the differential mechanism.
Japanese Patent Laid Open No. 55-83617 discloses such a transmitting system in which the distribution ratio of torque for the front wheels to the rear wheels is controlled by electronically controlling a central differential with a clutch or brake.
However, the system has only a central differential as a means for distributing the torque. Therefore, the distribution ratio is constant. In addition, the system can only select a two-wheel drive mode, four-wheel drive mode or a differential locking mode and is not intended for controlling the distribution ratio of torque.